1. Technical Field
The present invention relates to a Coriolis flowmeter, and more particularly to a Coriolis flowmeter including at least one flow tube constructed of a bent tube.
2. Background Art
In a Coriolis flowmeter, a tube through which a fluid to be measured flows is supported at one end or both ends thereof, and vibration is applied to a portion of the tube around the supporting point in a direction perpendicular to the flowing direction of the tube (hereinafter, a tube to which vibration is applied is referred to as a flow tube). The Coriolis flowmeter is a mass flowmeter, which utilizes the fact that the Coriolis forces applied to the flow tube when vibration is thus applied thereto, are proportional to a mass flow rate. The Coriolis flowmeter, which is well known, is roughly classified into two types in terms of f low tube structure: a straight tube type and a bent tube type.
In a Coriolis flowmeter of the straight tube type, when vibration is applied to a straight tube, whose both ends are supported, in a direction perpendicular to the straight-tube center portion axis, a difference in displacement due to the Coriolis forces is generated between the support portions and the central portion of the straight tube (that is, a phase difference signal is obtained), and, based on this phase difference signal, the mass flow rate is detected. The straight tube type Coriolis flowmeter thus constructed has a simple, compact, and solid structure. On the other hand, there arises a problem in that it is difficult to achieve high detection sensitivity.
In contrast, the bent tube type Coriolis flowmeter is superior to the straight tube type Coriolis flowmeter from a view point that it allows selection of a shape for effectively obtaining the Coriolis forces. In fact, it is capable of performing mass flowrate detection with high sensitivity. Known examples of the bent tube type Coriolis flowmeter include one equipped with a single flow tube (see, for example, JP 4-55250 B), one equipped with two flow tubes arranged in parallel (see, for example, JP 2939242 C), and one equipped with a single flow tube in a looped state (see, for example, JP 2951651 C).
However, of bent tube type Coriolis flowmeters, one with a single flow tube is advantageously capable of providing a mass flowmeter in the simplest structure and at low cost. On the other hand, it has the following problem. Since there is provided only one flow tube, when vibrating the flow tube, it is impossible to secure the vibration balance as in the case of a flowmeter with two flow tubes, making it impossible to obtain a stable signal.